The invention relates to a power-factor correction arrangement in which an active source of variable inductance and a passive source of capacitance are switchably disposed in parallel across a source of AC power, the passive device being arranged to be switched into circuit after the active device.
One example of a known power-factor correction arrangement is shown in FIG. 1 and is based on the disclosure of UK patent GB 2,167,582 filed in the name of the General Electric Company plc and published on May 29, 1986. In this arrangement a source of AC power, e.g. an 11 kV bus 10, feeds in parallel through respective AC circuit breakers 11, 12 and 13 a pair of loads 14, an active source of variable inductance 15 and a filter bank 16. These items involve conventional three-phase circuits, though only one phase is shown in the diagram. The loads in this example are constituted by a pair of DC motors 17 fed from a pair of thyristor convertors 18 which in turn are supplied with power from the bus via transformers 19. The variable-inductance source 15 comprises essentially a passive inductor 20 connected to a pair of series-connected thyristor bridge convertors 21 which in turn are fed from the separate secondaries of a transformer 22. The convertors control the firing of the thyristors by way of a multipulse output such as to provide in the stage 15 a current of variable lagging phase, this current flowing through the AC power bus 10. The filter bank 16 is in three stages, each designed to attenuate a particular harmonic of the AC source frequency but also to provide at that source frequency a net capacitive reactance, i.e. the filter appears as a leading-phase branch across the supply 10.
In one mode of operation of this arrangement, the filter 16 is arranged to provide leading current to fully compensate the full-load lagging reactive power of the loads 14. At less than full load, however, the capacitors in the filter bank 16 overcompensate and would give rise to a net leading reactive power in the system, were it not for the fact that the variable-inductance stage 15 is arranged to provide further lagging VARs (volt-amps reactive) to make up for the shortfall of lagging VARs in the motors. Thus, the lagging current in stage 15 and that in the load combine at all values of loading to equal the leading current in the filter bank 16, thereby giving rise to a substantially unity power factor.
In an alternative, and commonly employed, mode of operation the filter bank is switchable by additional circuits (not shown) between different values of capacitance such as to provide incremental changes in leading VARs to suit widely differing load conditions, the variable-inductance stage 15 then being controlled as before to provide zero net power factor. In other circumstances (for example, when the loads 14 are not in use for a significant period) the filter stage may need to be switched out of circuit together with the stage 15 in order to save energy. When the filter stage is switched in, there is found to occur a large pulse of current through the filter, followed by a large voltage surge which affects the filter components, the waveform of the AC power source and all other circuits connection to the bus 10. These surges can cause significant stress to the filter capacitors and other circuits and lead to the necessity to limit the switching rate of the filter stage 16 to a rate which is unacceptably low.
Waveforms relating to the power-factor correction arrangement just described are shown in FIG. 2. In FIG. 2, at a time 1.09 s approximately, the filter breaker 13 is closed, giving rise to a period in which a surge current 40 flows through the filter. FIG. 2 shows the three AC currents flowing into the filter stage which all start at the approximately 1.09 s point. There will be three corresponding AC voltage in the AC power system 10, but only the worst-affected of these is shown to aid clarity. At the same point in time, the supply voltage waveform 41 experiences a pronounced dip 42, followed approximately 10 ms later by a large voltage rise 43 amounting to an approximately 54% increase over normal peak voltage levels.
One known way of dealing with the undesirable current surge is illustrated in FIG. 3. In FIG. 3 the AC circuit breaker 13 is bypassed by a resistor 23 in series with an additional AC circuit breaker 24. Now, when the filter bank is due to be switched (it is assumed that breaker 12 is closed), breaker 24 is closed with breaker 13 open, so that the filter stage 16 is connected to the supply via the resistor 23, this serving to reduce the current surge. A short time later, breaker 13 is closed to fully energize the filter stage. A drawback with this approach, however, is the need for the further circuit breaker 24 (there will be one per phase). This component is not only expensive, it also takes up space and may in practice be difficult to retrofit on an existing control panel.
In a second known technique for minimizing filter current pulses during switching, the standard circuit breaker 13 is replaced by a special device having three independent contacts, or poles, operated by a special control arrangement. In operation, when the filter is to be switched into circuit, the first two poles are closed when the supply voltage is at a zero value and the third is then closed a few milliseconds later. Waveforms analogous to those of FIG. 2 but relating to this technique are shown in FIG. 4. This figure shows the point of closure 44 of the first two poles and that of the third (45) very shortly afterwards. In FIG. 4 only that voltage waveform is shown which is worst affected (43). The waveform which causes the poles to close at 44 is not shown. It can be seen that, though the levels of the current and voltage surges are reduced when compared with the basic arrangement of FIG. 1, they are still quite appreciable.
While this second technique is partially effective in reducing the undesired surges through the filter, it requires the use of an expensive, non-standard circuit breaker which, as in the case of the first solution, may be difficult to accommodate in already existing control equipment.
In accordance with the present invention, there is provided a power-factor correction arrangement, comprising an active source of variable inductance and a passive source of capacitance, the active and passive sources being connected to a source of AC power by way of respective first and second switching means, the arrangement being configured to close the second switching means while the first switching means is in a closed state, the active and passive sources being interconnected at their switched ends by means of a resistance.
Preferably, the resistance is chosen to have a value such that a magnitude of a current in the passive source during a closed state of the first switching means suffers substantially no change following closure of the second switching means.
The passive source may be constituted by one or more capacitors in combination, being either effectively pure capacitance or an inductance-capacitance combination forming a filter arrangement.
The active source may be constituted by, for example, a thyristor-controlled reactor or a pair of series-connected multipulse thyristor bridges.